A precursor pre-adsorbed sodium-supplemented sodium iron phosphate positive electrode material, a preparation method and a sodium ion battery

Abstract

The application provides a preparation method of a precursor pre-adsorption sodium-supplemented sodium iron phosphate pyrophosphate positive electrode material, uses hydrated iron phosphate as a precursor, and places the precursor in a sodium salt of an organic weak acid sodium supplement solution for impregnation adsorption, and after drying, is mixed with a sodium source, a phosphoric acid source, a pyrophosphoric acid source and a carbon source, and is sintered in an inert atmosphere to obtain a pre-sodium-supplemented Na4Fe3(PO4)2P2O7 / C material. The application also provides a sodium iron phosphate pyrophosphate positive electrode material prepared by using the above method, and a sodium ion battery comprising the material. By impregnating and adsorbing the sodium salt of the organic weak acid sodium supplement on the industrial precursor of the sodium iron phosphate pyrophosphate, the nanoscale dispersion of the sodium supplement is achieved, the toxicity and high gas production of the traditional sodium supplement are avoided from the root, the process is adapted to the existing industrial production line, the first coulomb efficiency and the cycle stability of the sodium iron phosphate pyrophosphate positive electrode material are effectively improved, and the industrial application is promoted.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery electrode materials technology, and in particular to a sodium iron pyrophosphate cathode material with precursor pre-adsorption and sodium replenishment, its preparation method, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries have significant application prospects in large-scale energy storage due to their abundant resources, low cost, and high safety. Among them, sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, abbreviated as NFPP) is a typical polyanionic sodium-ion battery cathode material, which has the characteristics of moderate working voltage, stable crystal structure, long cycle life, and low raw material cost, making it a cathode system with great industrialization potential.

[0003] However, NFPP materials exhibit low initial coulombic efficiency during the first charge-discharge cycle. This is primarily due to irreversible sodium ion loss caused by factors such as the formation of the solid electrolyte interphase (SEI) film on the negative electrode surface, irreversible electrolyte decomposition, material lattice defects, and interfacial side reactions. Low initial coulombic efficiency directly reduces the energy density of sodium-ion full cells, increases manufacturing costs, and limits the compatibility of battery systems.

[0004] At present, the industry mainly improves the first coulombic efficiency of NFPP materials by external sodium supplementation, that is, by introducing sodium supplementation agents that can release sodium ions in advance into the cathode system. However, the existing sodium supplementation technology still has many shortcomings: (1) Sodium supplementation agents and finished NFPP cathode materials are mostly simple physical blends, which are prone to uneven dispersion and agglomeration, resulting in a decrease in electrode compaction density and material cycle stability; (2) Sodium supplementation agents such as NaN3 and NaBH4 are toxic and generate serious gas during the first charge, posing significant safety hazards; (3) Conventional sodium supplementation agents such as Na2CO3 and Na2C2O4 release CO2 and other gases during the first charge, which can easily cause battery swelling and affect battery performance and safety; (4) Most sodium supplementation processes are not combined with the precursor system of NFPP industrial production, making it difficult to adapt to the existing mass production process and limiting industrial application.

[0005] In summary, developing a sodium replenishment technology for NFPP materials that features uniform sodium dispersibility, safety, low gas content, simple process, and suitability for industrial production is of great significance for promoting the industrialization of this cathode material. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide a sodium iron pyrophosphate cathode material with precursor pre-adsorption and sodium supplementation, a preparation method, and a sodium-ion battery. By impregnating and adsorbing an organic weak acid sodium salt sodium supplementer in the precursor stage of sodium iron pyrophosphate industrialization, the sodium supplementer is dispersed at the nanoscale, and the toxicity and high gas production problems of traditional sodium supplementers are avoided from the root. At the same time, the process is highly compatible with existing industrial production lines, effectively improving the first coulombic efficiency and cycle stability of the sodium iron pyrophosphate cathode material, and promoting its industrial application.

[0007] The present invention solves the above-mentioned technical problems by adopting the following technical solutions: A precursor-pre-adsorbed sodium-iron phosphate pyrophosphate sodium cathode material, using hydrated iron phosphate (FePO4) Using 2H2O as a precursor, the precursor was impregnated and adsorbed in an organic weak acid sodium salt sodium supplementation solution. After drying, it was mixed with sodium source, phosphoric acid source, pyrophosphate source and carbon source, and sintered under an inert atmosphere to obtain pre-sodium supplemented Na4Fe3(PO4)2P2O7 / C material.

[0008] As one of the preferred embodiments of the present invention, the organic weak acid sodium salt sodium supplement is selected from one or more of sodium glycolate, sodium gluconate, sodium lactate, sodium malonate, sodium succinate, and sodium glycerate.

[0009] As one of the preferred embodiments of the present invention, the amount of sodium supplement added is 1wt% to 5wt% of the theoretical mass of Na4Fe3(PO4)2P2O7.

[0010] As one of the preferred embodiments of the present invention, the impregnation and adsorption time is 1h to 3h.

[0011] As one of the preferred embodiments of the present invention, the sintering temperature is 680℃~750℃ and the sintering time is 6h~8h.

[0012] As one of the preferred embodiments of the present invention, the sodium source is selected from one or more of sodium carbonate, sodium oxalate, and sodium acetate.

[0013] As one of the preferred embodiments of the present invention, the phosphoric acid source is ammonium dihydrogen phosphate.

[0014] As one of the preferred embodiments of the present invention, the pyrophosphate source is selected from one or both of ammonium dihydrogen pyrophosphate and pyrophosphate.

[0015] As one of the preferred embodiments of the present invention, the carbon source is selected from one or more of glucose, sucrose, and pitch.

[0016] A sodium iron pyrophosphate cathode material with precursor pre-adsorption and sodium supplementation is prepared by the above-mentioned preparation method.

[0017] A sodium-ion battery comprising the aforementioned sodium iron pyrophosphate cathode material.

[0018] The advantages of this invention compared to the prior art are: (1) This invention carries out sodium supplementation agent pre-adsorption treatment in the hydrated iron phosphate precursor stage, so that the sodium supplementation agent is uniformly distributed in the form of surface adsorption and pore loading, and realizes the nanoscale dispersion of sodium supplementation agent in the material system. Compared with the traditional method of physical blending of finished cathode and sodium supplementation agent, this invention effectively avoids the problems of sodium supplementation agent agglomeration and segregation, so that the sodium ion release during the first charge and discharge cycle is more uniform and the compensation is more sufficient, and the first coulombic efficiency of sodium iron pyrophosphate cathode is greatly improved.

[0019] (2) The present invention uses sodium hydroxide, sodium gluconate, sodium lactate, sodium malonate, sodium succinate, sodium glycerate and other organic weak acid sodium salts to construct the sodium replenishment system, and abandons highly toxic and strongly reactive sodium replenishment agents such as sodium azide and sodium borohydride, and does not use conventional high gas production sodium replenishment agents such as sodium carbonate and sodium oxalate; the sodium replenishment agent of the present invention decomposes stably during the first week of activation, without the release of toxic gas or obvious gas production bulging phenomenon, which significantly improves the safety of battery preparation and use, and at the same time avoids the gas from damaging the electrode structure, interface stability and current collector.

[0020] (3) This invention uses hydrated iron phosphate, which is commonly used in the industrial production of sodium iron pyrophosphate, as a precursor. Only the impregnation adsorption and drying steps are added before sintering. There is no need to change the core process flow such as main material preparation, sand milling, and sintering. It is highly compatible with existing cathode material industrial production lines. The method is simple, easy to operate, mild, and easy to scale up. It can achieve efficient sodium supplementation without significantly increasing production costs and has good industrialization prospects.

[0021] (4) The in-situ sodium replenishment achieved by the precursor pre-adsorption of the present invention, on the basis of efficient sodium replenishment and improved first coulombic efficiency, does not introduce obvious impurity phases, does not destroy the crystal structure and morphology of sodium iron pyrophosphate, and does not reduce the electronic conductivity and ion transport capacity of the material. The prepared Na4Fe3(PO4)2P2O7 / C composite material has excellent cycle stability, slow capacity decay and structural integrity during long cycle, and can meet the performance requirements of high first efficiency, high rate and long life. The comprehensive electrochemical performance is significantly better than the traditional sodium replenishment method.

[0022] (5) The precursor pre-adsorption strategy of the present invention allows the sodium supplement to form a stable interface in situ with the cathode material during the subsequent sintering process, reducing interfacial side reactions and irreversible capacity loss, and further optimizing the electrode-electrolyte interface compatibility. Compared with the traditional method of directly adding sodium supplement, the present invention can effectively reduce interfacial impedance and improve charge transport efficiency, so that the rate performance, voltage stability and cycle consistency of the full cell are significantly improved. Attached Figure Description

[0023] Figure 1 This is the XRD diffraction pattern of the pre-supplemented sodium-type Na4Fe3(PO4)2P2O7 / C composite material of Example 1 of the present invention; Figure 2 This is the XRD diffraction pattern of the blank sample without sodium supplementation in Comparative Example 2 of this invention; Figure 3 This is a SEM image (2.50kx magnification, scale bar 20.0μm) of the pre-supplemented sodium-type Na4Fe3(PO4)2P2O7 / C composite material in Example 1 of the present invention. Figure 4 This is a SEM image (20.0kx magnification, scale bar 2.00μm) of the sodium-added physical blend sample of Comparative Example 1 of this invention. Detailed Implementation

[0024] The embodiments of the present invention are described in detail below. These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0025] Furthermore, unless otherwise specified, all raw materials and reagents used in this invention are commercially available industrial-grade or battery-grade products; the drying, ball milling, sintering, and other operations involved all employ conventional process conditions and equipment in the field; the ambient humidity is strictly controlled to ≤50% during the operation process to prevent the raw materials from absorbing moisture and affecting product performance. Among them, hydrated iron phosphate (FePO4) The sodium supplement (2H2O) is battery grade, with a particle size D50 of 1.5~3.0μm; all sodium supplements are analytical grade, and the sodium source, phosphoric acid source, pyrophosphate source, and carbon source are all battery grade; all solutions are prepared using deionized water (conductivity ≤10μS / cm).

[0026] Example 1 The preparation method of a sodium iron pyrophosphate cathode material in this embodiment is as follows: (1) Weigh out the battery-grade hydrated iron phosphate precursor (FePO4). 15.00 g of 2H₂O was placed in a vacuum drying oven and dried for 2 hours at 80℃ and a vacuum of -0.08 MPa to remove the moisture adsorbed on the surface of the precursor. After cooling to room temperature, it was ready for use. The pretreated precursor was free of lumps and impurities and had a uniform particle size.

[0027] (2) Take a clean 500mL beaker, add 60.0mL of deionized water, place it on a magnetic stirrer, adjust the stirring speed to 300r / min, slowly add sodium glycolate (analytical grade, purity ≥99.8%, added at 1wt% of the theoretical mass of Na4Fe3(PO4)2P2O7), and continue stirring for 15min until the sodium glycolate is completely dissolved, to obtain a uniform transparent sodium supplement aqueous solution with a mass fraction of 1%, without undissolved particles, for later use.

[0028] (3) The pretreated FePO4 The 2H2O precursor was slowly added to the above sodium supplement aqueous solution. The magnetic stirrer speed was adjusted to 250 r / min. Under the condition of maintaining room temperature (25±2℃), the soaking and adsorption were continued for 1 h. During this period, the mixture was stirred for 5 min every 15 min to ensure that the sodium supplement fully penetrated and adsorbed on the surface and internal pore structure of the precursor particles. After the adsorption was completed, a uniform suspension was obtained without obvious precipitation and stratification.

[0029] (4) After adsorption, the suspension was transferred to a spray dryer. The spray drying parameters were set as follows: inlet temperature 180℃, outlet temperature 85℃, feed rate 5mL / min, and atomization pressure 0.2MPa. The suspension was then spray dried to obtain dried FePO4 loaded with sodium supplementation agent. 2H2O precursor powder. The dried powder is light gray, free of lumps, with a D50 of 2.0~3.5μm, and is ready for use.

[0030] (5) Accurately weigh 15.30 g of dried sodium ferric pyrophosphate (Na4Fe3(PO4)2P2O7) according to its stoichiometric ratio. 2H₂O precursor powder, 4.82g sodium oxalate, 5.13g ammonium dihydrogen phosphate, 3.28g ammonium dihydrogen pyrophosphate, and 1.20g glucose. Place all the above ingredients in an agate mortar and grind manually for 10 minutes to initially mix them evenly. Then transfer them to a planetary ball mill jar for later use.

[0031] (6) Add 50 mL of deionized water as a dispersion medium to the planetary ball mill jar, add agate balls (ball-to-material ratio = 8:1, agate ball particle size 5 mm), and set the ball milling parameters: rotation speed 400 r / min, ball milling time 4 h, with a 10 min pause every 1 h to avoid overheating of the ball mill jar. After ball milling, a uniform and fine slurry is obtained, with a slurry particle size D50 ≤ 1.0 μm and no obvious large particles.

[0032] (7) The ball-milled slurry was transferred to a vacuum drying oven and dried at 100℃ and a vacuum of -0.09MPa for 8 hours to remove moisture from the slurry and obtain a dry mixed powder. The mixed powder was loaded into an alumina crucible, compacted, and placed in a tube furnace. Argon gas (purity ≥99.999%) was introduced as an inert protective atmosphere, and the argon gas flow rate was controlled at 50mL / min. The temperature was first raised from room temperature to 300℃ at a heating rate of 5℃ / min and held for 2 hours to remove crystal water and organic impurities from the raw material. Then, the temperature was raised to 680℃ at a heating rate of 3℃ / min and held for 8 hours to carry out a high-temperature sintering reaction. After sintering, the temperature was cooled to room temperature with the furnace at a cooling rate of 2℃ / min to obtain a pre-supplemented sodium-type Na4Fe3(PO4)2P2O7 / C cathode composite material.

[0033] Example 2 The preparation method of a sodium iron pyrophosphate cathode material in this embodiment is as follows: (1) Weigh out the battery-grade hydrated iron phosphate precursor (FePO4). 15.00 g of 2H₂O was placed in a vacuum drying oven and dried for 2 hours at 80℃ and a vacuum of -0.08 MPa to remove the moisture adsorbed on the surface of the precursor. After cooling to room temperature, it was ready for use. The pretreated precursor was free of lumps and impurities and had a uniform particle size.

[0034] (2) Take a clean 500mL beaker, add 60.0mL of deionized water, place it on a magnetic stirrer, adjust the stirring speed to 300r / min, add sodium glycolate and sodium gluconate respectively, and continue stirring for 20min until the two sodium supplements are completely dissolved, to obtain a composite sodium supplement aqueous solution with a mass ratio of sodium glycolate and sodium gluconate of 1:1 (the total amount of sodium glycolate and sodium gluconate is 2wt% of the theoretical mass of Na4Fe3(PO4)2P2O7), which is uniform and transparent, without undissolved impurities, and is ready for use.

[0035] (3) The pretreated FePO4 The 2H2O precursor was slowly added to the above-mentioned composite sodium supplement aqueous solution. The magnetic stirrer speed was adjusted to 250 r / min. Under the condition of maintaining room temperature (25±2℃), the mixture was continuously immersed and adsorbed for 1 h. During this period, the mixture was stirred for 5 min every 15 min to ensure that the composite sodium supplement was uniformly adsorbed on the surface and in the pores of the precursor. After the adsorption was completed, a uniform suspension was obtained.

[0036] (4) After adsorption, the suspension is transferred to a spray dryer and the spray drying parameters are set as follows: inlet temperature 180℃, outlet temperature 85℃, feed rate 5mL / min, atomization pressure 0.2MPa. Spray drying is performed. After drying, a light gray, non-clumped sodium-loaded precursor powder with particle size D50 = 2.0~3.5μm is obtained for later use.

[0037] (5) Accurately weigh 15.60 g of dried FePO4 containing the composite sodium supplement according to the stoichiometric ratio of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7). 2H₂O precursor powder, 4.82g sodium oxalate, 5.13g ammonium dihydrogen phosphate, 3.28g ammonium dihydrogen pyrophosphate, and 1.20g glucose. Grind all the above ingredients manually in an agate mortar for 10 minutes, then transfer to a planetary ball mill jar for later use.

[0038] (6) Add 50 mL of deionized water as a dispersion medium to the planetary ball mill jar, add agate balls (ball-to-material ratio = 8:1, agate ball particle size 5 mm), and set the ball milling parameters: rotation speed 400 r / min, ball milling time 4 h, with a 10 min pause every 1 h to avoid overheating of the ball mill jar. After ball milling, a uniform and fine slurry is obtained, with a slurry particle size D50 ≤ 1.0 μm and no obvious large particles.

[0039] (7) The ball-milled slurry was transferred to a vacuum drying oven and dried at 100℃ and a vacuum of -0.09MPa for 8 hours to remove moisture from the slurry and obtain a dry mixed powder. The mixed powder was loaded into an alumina crucible, compacted, and placed in a tube furnace. Argon gas (purity ≥99.999%) was introduced as an inert protective atmosphere, and the argon gas flow rate was controlled at 50mL / min. The temperature was first raised from room temperature to 300℃ at a heating rate of 5℃ / min and held for 2 hours to remove crystal water and organic impurities from the raw material. Then, the temperature was raised to 680℃ at a heating rate of 3℃ / min and held for 8 hours to carry out a high-temperature sintering reaction. After sintering, the temperature was cooled to room temperature with the furnace at a cooling rate of 2℃ / min to obtain a pre-supplemented sodium-type Na4Fe3(PO4)2P2O7 / C cathode composite material.

[0040] Example 3 The preparation method of a sodium iron pyrophosphate cathode material in this embodiment is as follows: (1) Weigh out the battery-grade hydrated iron phosphate precursor (FePO4). 15.00 g of 2H₂O was placed in a vacuum drying oven and dried for 2 hours at 80℃ and a vacuum of -0.08 MPa to remove the moisture adsorbed on the surface of the precursor. After cooling to room temperature, it was ready for use. The pretreated precursor was free of lumps and impurities and had a uniform particle size.

[0041] (2) Take a clean 500mL beaker, add 60.0mL of deionized water, place it on a magnetic stirrer, adjust the stirring speed to 300r / min, slowly add sodium lactate (add 3wt% of the theoretical mass of Na4Fe3(PO4)2P2O7), continue stirring for 20min until completely dissolved, and obtain a sodium lactate aqueous solution that is uniform and transparent, for later use.

[0042] (3) The pretreated FePO4 The 2H2O precursor was slowly added to the above sodium lactate aqueous solution. The magnetic stirrer speed was adjusted to 250 r / min. Under the condition of maintaining room temperature (25±2℃), the soaking and adsorption were continued for 1.5 h. During this period, the mixture was stirred for 5 min every 20 min to ensure that the sodium lactate was fully adsorbed on the surface and in the pores of the precursor. After the adsorption was completed, a uniform suspension was obtained.

[0043] (4) Vacuum drying was used. The adsorbed suspension was transferred to a vacuum drying oven and dried at 80°C and a vacuum of -0.08 MPa for 12 hours. During the drying process, the suspension was stirred every 2 hours to prevent clumping. After drying, FePO4 loaded with sodium lactate was obtained. 2H2O precursor powder, light gray, non-caking, particle size D50=2.2~3.8μm, for later use.

[0044] (5) Accurately weigh 16.80 g of dried sodium lactate-loaded FePO4 according to the stoichiometric ratio of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7). 2H₂O precursor powder, 5.10g sodium acetate, 5.13g ammonium dihydrogen phosphate, 2.85g pyrophosphate, and 1.20g sucrose. Grind all the above ingredients manually in an agate mortar for 10 minutes, then transfer to a planetary ball mill jar for later use.

[0045] (6) Add 50 mL of deionized water as a dispersion medium to the planetary ball mill jar, add agate balls (ball-to-material ratio = 8:1, agate ball particle size 5 mm), and set the ball milling parameters: rotation speed 400 r / min, ball milling time 4 h, with a 10 min pause every 1 h to avoid overheating of the ball mill jar. After ball milling, a uniform and fine slurry is obtained, with a slurry particle size D50 ≤ 1.0 μm and no obvious large particles.

[0046] (7) The ball-milled slurry was vacuum dried at 100℃ for 8 hours, loaded into an alumina crucible, placed in a tube furnace, protected by argon gas (flow rate 50 mL / min), heated to 350℃ at 5℃ / min and held for 2 hours; then heated to 700℃ at 3℃ / min and held for 7 hours; cooled to room temperature with the furnace to obtain the pre-supplemented sodium type Na4Fe3(PO4)2P2O7 / C cathode composite material.

[0047] Example 4 The preparation method of a sodium iron pyrophosphate cathode material in this embodiment is as follows: (1) Weigh out the battery-grade hydrated iron phosphate precursor (FePO4). 15.00 g of 2H₂O was placed in a vacuum drying oven and dried for 2 hours at 80℃ and a vacuum of -0.08 MPa to remove the moisture adsorbed on the surface of the precursor. After cooling to room temperature, it was ready for use. The pretreated precursor was free of lumps and impurities and had a uniform particle size.

[0048] (2) Take a clean 500mL beaker, add 60.0mL of deionized water, place it on a magnetic stirrer, adjust the stirring speed to 300r / min, slowly add sodium succinate (add according to 5wt% of the theoretical mass of Na4Fe3(PO4)2P2O7), continue stirring for 25min until completely dissolved, and obtain sodium succinate aqueous solution, which is uniform and transparent, for later use.

[0049] (3) The pretreated FePO4 The 2H2O precursor was slowly added to the above sodium succinate aqueous solution. The magnetic stirrer speed was adjusted to 250 r / min. Under the condition of maintaining room temperature (25±2℃), the immersion adsorption was continued for 3.0 h. During this period, the mixture was stirred for 5 min every 30 min to ensure that the sodium succinate was fully adsorbed and penetrated into the depth of the precursor pores. After the adsorption was completed, a uniform suspension was obtained.

[0050] (4) After adsorption, the suspension is transferred to a spray dryer and the spray drying parameters are set as follows: inlet temperature 180℃, outlet temperature 85℃, feed rate 5mL / min, atomization pressure 0.2MPa. Spray drying is performed. After drying, sodium succinate precursor powder is obtained without agglomeration and D50 = 2.0~3.5μm. It is ready for use.

[0051] (5) Accurately weigh 18.00 g of dried sodium succinate-loaded FePO4 according to the stoichiometric ratio of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7). 2H₂O precursor powder, 4.24g sodium carbonate, 5.13g ammonium dihydrogen phosphate, 3.28g ammonium dihydrogen pyrophosphate, and 1.20g pitch. Grind all the above materials manually in an agate mortar for 10 minutes, then transfer to a planetary ball mill jar for later use.

[0052] (6) Add 50 mL of deionized water as a dispersion medium to the planetary ball mill jar, add agate balls (ball-to-material ratio = 8:1, agate ball particle size 5 mm), and set the ball milling parameters: rotation speed 400 r / min, ball milling time 4 h, with a 10 min pause every 1 h to avoid overheating of the ball mill jar. After ball milling, a uniform and fine slurry is obtained, with a slurry particle size D50 ≤ 1.0 μm and no obvious large particles.

[0053] (7) The slurry was vacuum dried at 100℃ for 8h, loaded into an alumina crucible, and protected with argon gas in a tube furnace (flow rate 50mL / min). The temperature was increased to 300℃ at 5℃ / min and held for 2h; then increased to 750℃ at 3℃ / min and held for 6h; and cooled to room temperature with the furnace to obtain the pre-supplemented sodium type Na4Fe3(PO4)2P2O7 / C cathode composite material.

[0054] Comparative Example 1 The sodium iron pyrophosphate cathode material in this comparative example is basically the same as that in Example 1, except that: instead of impregnating and adsorbing the sodium supplementing agent in the hydrated iron phosphate precursor stage, the sodium glycolate sodium supplementing agent is physically blended with the prepared sodium iron pyrophosphate cathode material to supplement sodium.

[0055] The physical blending sodium supplementation process is as follows: Weigh the prepared unsodium-supplemented Na4Fe3(PO4)2P2O7 / C finished material, add sodium glycolate (the type and amount of sodium supplementation agent are exactly the same as in Example 1), put it in an agate mortar, grind manually for 30 minutes, and then put it in a planetary ball mill jar for dry ball milling for 2 hours (400 r / min) to fully mix the sodium supplementation agent with the finished cathode material, and obtain the physically blended sodium-supplemented sodium pyrophosphate cathode material.

[0056] Comparative Example 2 This comparative example of sodium iron pyrophosphate cathode material is basically the same as that in Example 1, except that no sodium supplement is added throughout the process, there is no sodium supplement impregnation or adsorption, or any other sodium supplementation-related operations, and the sodium iron pyrophosphate cathode material without sodium supplement is directly prepared.

[0057] Comparative Example 3 The sodium iron pyrophosphate cathode material in this comparative example is basically the same as that in Example 1, except that the "organic weak acid sodium salt sodium supplementer - sodium glycolate" used for precursor impregnation and adsorption is replaced with "inorganic sodium supplementer - sodium carbonate". The amount of sodium supplementer added, impregnation and adsorption and all subsequent process parameters are consistent with those in Example 1.

[0058] Experimental Example This experiment is used to test the performance and characterize the sodium iron pyrophosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-3.

[0059] I. Testing and Characterization Methods The sodium iron pyrophosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-3 were subjected to electrochemical performance testing and microstructure and crystal form characterization, respectively. The specific methods are as follows: 1. Electrochemical performance testing The positive electrode materials, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 8:1:1. N-methylpyrrolidone (NMP) was added and stirred to form a uniform slurry. The slurry was coated on aluminum foil and vacuum dried at 120°C for 12 hours to form the positive electrode sheet.

[0060] The obtained positive electrode was placed in an argon-filled glove box to assemble a button cell, thus producing a half cell. The button cell model was CR2032. The negative electrode was a sodium metal sheet, the separator was glass fiber, and the electrolyte was a 1 mol / L NaClO4 solution (the solvent was a mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1:1 and 5% fluoroethylene carbonate).

[0061] The battery's initial discharge specific capacity and initial coulombic efficiency were tested at 25℃ and a charge / discharge rate of 0.1C; a 500-cycle test was conducted at a charge / discharge rate of 1C to calculate the cycle capacity retention rate; comparative example 3 additionally used a sealed coin cell to test the gas generation rate during the first charge.

[0062] 2. Microstructure and Crystal Form Characterization X-ray diffraction (XRD) was used to test the crystal structure and phase composition of the pre-sodium-added Na4Fe3(PO4)2P2O7 / C composite material prepared in Example 1 and the unadded sodium blank sample in Comparative Example 2. Scanning electron microscopy (SEM) was used to observe the microstructure of the pre-sodium-added Na4Fe3(PO4)2P2O7 / C composite material prepared in Example 1 and the physically blended sodium-added sample in Comparative Example 1, and to analyze the particle distribution, morphological characteristics and dispersion state of the sodium supplement.

[0063] II. Test and Characterization Results 1. Electrochemical performance test results The electrochemical performance test results of each embodiment and comparative example are shown in Table 1.

[0064] Table 1. Electrochemical performance indicators of batteries assembled with various cathode materials

[0065] 2. Microstructure and Crystal Form Characterization Results (1) XRD diffraction pattern analysis: The characteristic diffraction peaks of the pre-supplemented sodium-type Na4Fe3(PO4)2P2O7 / C composite material prepared in Example 1 are sharp and the baseline is stable. The positions of each diffraction peak are completely matched with the standard PDF card of Na4Fe3(PO4)2P2O7. No obvious impurity phase peaks appear. Figure 1This indicates that the precursor pre-adsorption sodium replenishment process of the present invention did not destroy the crystal structure of sodium iron pyrophosphate, and the material has a complete crystal form and high purity; while the XRD pattern of the blank sample without sodium replenishment in Comparative Example 2 has some weak impurity phase diffraction peaks, and the characteristic peak intensity is weak and the baseline background value is high. Figure 2 This indicates that the material has low crystallinity and poor crystal integrity when sodium is not added. This not only introduces structural impurities but also increases the resistance to sodium ion transport, which is one of the reasons for its poor electrochemical performance.

[0066] (2) SEM morphology analysis: The SEM morphology of the pre-sodium-supplemented Na4Fe3(PO4)2P2O7 / C composite material prepared in Example 1 shows that the material particles are spherical or irregular polyhedral in shape, with concentrated and uniform particle size distribution. The surface and internal pore structure of the particles are clear, and there are no large particles or local enrichment areas formed by obvious sodium-supplementing agent agglomeration. Figure 3 This indicates that the sodium supplement achieved uniform nanoscale dispersion through precursor pre-adsorption, without any agglomeration of the sodium supplement; however, the SEM morphology of the physically blended sodium supplement sample in Comparative Example 1 showed obvious irregular deposits on the surface of the material particles, and large particles formed by agglomeration of the sodium supplement in some areas, with a discrete particle size distribution. Figure 4 This demonstrates that traditional physical blending methods cannot achieve uniform dispersion of sodium supplementation agents, which can easily lead to localized accumulation of sodium supplementation agents, thereby affecting the electrochemical performance of the materials.

[0067] III. Results Analysis The electrochemical performance test and microstructure characterization results show that the sodium replenishment technology of the present invention, which uses precursor pre-adsorption of organic weak acid sodium salt, exhibits significant advantages in crystal structure, microstructure, and electrochemical performance, as detailed below: 1. Complete crystal structure with no impurity phases introduced. The XRD characterization results of Example 1 show that the precursor pre-adsorption sodium replenishment process achieves efficient sodium replenishment without destroying the crystal structure of sodium iron pyrophosphate or generating additional impurity phases, thus ensuring the structural stability of the material itself and laying the foundation for excellent electrochemical performance. In contrast, Comparative Example 2 without sodium replenishment has a small number of crystal defects, resulting in decreased ion transport capacity and significantly inferior first coulombic efficiency and cycle performance compared to the examples.

[0068] 2. The sodium supplement is evenly dispersed, improving interfacial stability. A comparison of SEM characterization of Example 1 and Comparative Example 1 shows that the precursor pre-adsorption method allows the sodium supplementer to be uniformly loaded on the surface and pores of the precursor. After sintering, the resulting composite material has a uniform particle distribution, no agglomeration, and lower electrode interface impedance. In contrast, the sodium supplementer in Comparative Example 1, which uses physical blending for sodium supplementation, is only mechanically mixed with the finished material, which cannot achieve nanoscale dispersion and is prone to local enrichment. This leads to the obstruction of sodium ion transport during charge and discharge, resulting in a significant reduction in its initial coulombic efficiency and cycle capacity retention.

[0069] 3. Significantly improved electrochemical performance, safe and low-gas content. The initial coulombic efficiency of each embodiment was consistently above 92.8%, an improvement of over 10 percentage points compared to Comparative Example 2 without sodium supplementation, and an improvement of over 5 percentage points compared to Comparative Example 1 with physical blending and sodium supplementation. The 1C / 500-cycle capacity retention rate was consistently above 89.8%, demonstrating that the sodium supplementation scheme of this invention can effectively compensate for the irreversible loss of sodium ions in the first cycle and significantly improve the cycle stability of the material. Furthermore, this invention uses an organic weak acid sodium salt as the sodium supplement agent. The initial charge gas generation rate of Example 1 was only 1.5%, while the gas generation rate of Comparative Example 3, which used the inorganic sodium supplement agent sodium carbonate, reached as high as 12.3%. This fully demonstrates the safety and low gas generation advantages of the sodium supplementation system of this invention, effectively avoiding battery gas generation and bulging problems, and improving safety in use.

[0070] 4. The compound sodium supplement has a synergistic effect and optimal performance. Comparison of Examples 1-4 shows that pure-phase Na4Fe3(PO4)2P2O7 / C composite materials can be obtained by using different types of organic weak acid sodium salts (sodium glycolate, sodium gluconate, sodium lactate, sodium succinate), different amounts of sodium supplement (1wt%~5wt%), different sodium sources, phosphorus sources, carbon sources, and different drying and sintering processes. The initial coulombic efficiency of the materials can be stably maintained within the range of 92.8%~94.0%, and the capacity retention rate after 500 cycles at 1C rate can reach 89.8%~91.3%, indicating that the present invention has good universality, stability, and adjustability. Among them, Example 2, which uses a composite sodium supplement of sodium glycolate and sodium gluconate, has the best initial discharge specific capacity, initial coulombic efficiency, and cycle capacity retention rate among all samples, demonstrating the synergistic effect of the composite sodium supplement in terms of sodium ion release rate and interfacial compatibility, and is the optimal implementation method of the present invention.

[0071] In summary, the sodium iron pyrophosphate cathode material prepared by the precursor pre-adsorption sodium replenishment process of the present invention has complete crystal structure, excellent microstructure, and uniform dispersion of sodium replenishment agent. It also has the characteristics of high initial coulombic efficiency, excellent cycle stability and safe low gas content, which are far superior to traditional sodium replenishment methods and cathode materials without sodium replenishment. It has good application prospects in the field of sodium-ion batteries for power and energy storage.

[0072] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a sodium iron pyrophosphate cathode material with precursor pre-adsorption and sodium supplementation, characterized in that, Using hydrated ferric phosphate as a precursor, the precursor was impregnated and adsorbed in an organic weak acid sodium salt sodium supplementation solution. After drying, it was mixed with sodium source, phosphate source, pyrophosphate source and carbon source, and sintered under an inert atmosphere to obtain pre-sodium supplemented Na4Fe3(PO4)2P2O7 / C material.

2. The preparation method according to claim 1, characterized in that, The organic weak acid sodium salt sodium supplement is selected from one or more of sodium glycolate, sodium gluconate, sodium lactate, sodium malonate, sodium succinate, and sodium glycerate.

3. The preparation method according to claim 1, characterized in that, The amount of sodium supplement added is 1 wt% to 5 wt% of the theoretical mass of Na4Fe3(PO4)2P2O7.

4. The preparation method according to claim 1, characterized in that, The impregnation and adsorption time is 1h to 3h.

5. The preparation method according to claim 1, characterized in that, The sintering temperature is 680℃~750℃, and the sintering time is 6h~8h.

6. The preparation method according to claim 1, characterized in that, The sodium source is selected from one or more of sodium carbonate, sodium oxalate, and sodium acetate.

7. The preparation method according to claim 1, characterized in that, The phosphoric acid source is ammonium dihydrogen phosphate, and the pyrophosphate source is selected from one or both of ammonium dihydrogen pyrophosphate and pyrophosphate.

8. The preparation method according to claim 1, characterized in that, The carbon source is selected from one or more of glucose, sucrose, and pitch.

9. A sodium iron pyrophosphate cathode material with precursor pre-adsorption and sodium replenishment, characterized in that, It is prepared by any one of the preparation methods described in claims 1 to 8.

10. A sodium-ion battery, characterized in that, Including the sodium iron pyrophosphate cathode material as described in claim 9.

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